This invention relates generally to load responsive fluid control valves and to fluid power systems incorporating such valves which systems are supplied by a single fixed or variable displacement pump. Such control valves are equipped with an automatic load responsive control, and can be used in a multiple load system in which a plurality of loads are individually controlled under positive and negative load conditions by separate control valves.
In more particular aspects this invention relates to direction and flow control valves capable of controlling simultaneously a number of loads under both positive and negative conditions.
Closed center load responsive fluid control valves are very desirable for a number of reasons. They permit load control with reduced power losses, and therefore increased system efficiency; and when controlling one load at a time they provide a feature of flow control, irrespective of the variation in the magnitude of the load. Normally such valves include a load responsive control which automatically maintains the pump discharge pressure at a level higher, by a constant pressure differential, than the pressure required to sustain the load. A variable orifice, introduced between pump and load, varies the flow supplied to the load, each orifice area corresponding to a different flow level, which is maintained constant, irrespective of variation in the magnitude of the load. The application of such a system is, however limited by several basic system disadvantages.
Since in this system the variable control orifice is located between the pump and the load, the control signal to a pressure regulating throttling device is at a high pressure level, inducing high forces in the control mechanism. Another disadvantage of such a control is that it regulates the flow of fluid into the motor and therefore does not compensate for fluid compressibility and leakage across both motor and valve. A fluid control valve for such a system is shown in U.S. Pat. No. 3,488,953 issued to Hausler.
The valve control can maintain a constant pressure differential and therefore constant flow characteristics when operating only one load at a time. With two or more loads, simultaneously controlled, only the highest of the loads will retain the flow control characteristics, the speed of actuation of the lower loads varying with the change in magnitude of the highest load. This drawback can be overcome in part by the provision of a proportional valve as disclosed in my U.S. Pat. No. 3,470,694, dated Oct. 7, 1969 and also in U.S. Pat. No. 3,455,210 issued to Allen on July 15, 1969. However, while these valves are effective in controlling positive loads they do not retain flow control characteristics when controlling negative loads, which instead of taking, supply the energy to the fluid system, and hence the speed of actuation of such a load in a negative load system will vary with the magnitude of the negative load. Especially with so-called overcenter loads, where a positive load may become a negative load, such a valve will lose its speed control characteristics in the negative mode.
This drawback can be overcome by provision of a load responsive fluid control valve as disclosed in my U.S. Pat. Nos. 3,744,517 issued July 10, 1973, 3,858,393 issued Jan. 7, 1975. However, while this valve is effective in controlling both positive and negative loads it still utilizes a controlling orifice located between the pump and the motor during positive load mode of operation and therefore controls the fluid flow into the fluid motor instead of controlling fluid flow out of the fluid motor, which approach carries distinct advantages.